The current invention relates to a salt analyzer, and more particularly to a salt analyzer switchably capable of use with both contact and non-contact conductivity probes.
The salinity of a solution is related to the electric conductivity of the solution. Consequently, the salinity or ion concentration of a solution may be determined by measuring the electric conductivity of the solution with devices known generally as salimeters or salt analyzers. These devices basically are comprised of an exciting AC voltage source, a pair of electrodes, and a circuit capable of measuring a current or voltage induced in the solution due to the exciting AC voltage. The pair of electrodes is either constituted in the form of a probe capable of being immersed directly into a solution or provided in a measuring cell into which the solution is sampled. The exciting AC voltage source supplies an AC voltage between the electrodes with the probe immersed in solution or with the solution sampled into a cell. In principle, the solution conductivity (and thus its ion concentration) is obtained from the data of voltage and current between the electrodes and the geometry of the pair of electrodes. The device is typically designed to give a resultant value of the conductivity and/or salinity of the solution based on the probe or cell constant reflecting the geometry of the pair of electrodes.
Generally the electrodes used with salt analyzers are one of two types. The first type is referred to as a "contact" probe, cell or sensor. With a contact probe conductivity of a solution is determined by measuring the current through the cell for a given voltage applied then multiplying by the cell constant appropriate for the cell geometry. As known by the skilled artisan, the cell constant is (electrode separation)/(cell cross-sectional area).) The second general type of sensing electrode is the "contactless" or "non-contact" probe, sensor or cell.
In it simplest form, a non-contact probe consists of two toroidal coils complied by a loop of the solution being tested. The non-contact probe operates by applying a voltage to the first coil, measuring the induced secondary voltage in the other coil and multiplying by the appropriate cell constant.
Salt analyzers capable of employing either contact probes or non-contact probes are well-known in the art. In particular, the art focuses on refinements to overcome various technical problems recurrent with either contact or non-contact probes or to improve the operation of such probes. For example, U.S. Pat. No. 4,227,151 discloses a contact-type electrical measuring cell comprising at least four (4) concentric circular electrodes separated by annular areas and adapted to receive a temperature sensitive element. As part of the disclosure of U.S. Pat. No. 4,227,151 a single electrical system is taught to measure the conductivity of both the "measured liquor" and the "reference liquor" by employing suitable switching means. In U.S. Pat. No. 3,979,665 a conductivity monitoring system having a temperature compensation means and an arrangement for indicating failure of that compensating means is disclosed. Improvements to non-contact probe systems are disclosed, inter alia, in U.S. Pat. No. 4,825,168 which teaches the use of a square wave excitation signal in the drive transformer and in WO 91/0600 which teaches a non-contact measuring cell having three toroids with a switching control to allow conversion to a conventional two toroid system or to change the drive and sensing toroids.
Each type of probe, contact and non-contact, has its own limitations and advantages. A contact probe is prone to contamination at the electrode surface, requiring periodic cleaning and, therefore, presenting a maintenance problem in e.g. pipeline installations. A contact probe can also experience electrode-to-liquid interface impedance introducing errors when measuring low resistance (high conductivity) solutions. However, a contact probe is particularly useful in laboratory scale bench-top applications due to its relative independence from the container holding the tested solution.
A non-contact probe is less sensitive to coating and surface contamination, requiring less maintenance than a contact probe. This makes such probes better suited to applications having difficult accessibility. On the other hand, since all the liquid surrounding a non-contact probe forms part of the conductive path, the effective geometry is not as well defined as that of a contact probe. Consequently use of a non-contact probe as a "dip-in" probe is problematic since the depth of submersion will affect the measurement. Additionally, errors will be introduced if the conductive path is distorted by the container bottom or walls.
Clearly, employment of contact probes or non-contact probes changes from application to application and environment to environment. However, current salt analyzers are dedicated to either contact or non-contact probes since, at their most basic level, contact probes detect current and non-contact probes detect induced voltage. The current invention provides, among other things, a salt analyzer capable of use with both contact and non contact probes.